It has heretofore been widely practiced to mix an inorganic filler with a resin for the purpose of improving or enhancing the characteristics of resin materials. For example, a method has been proposed which comprises mixing various inorganic fillers such as inorganic fibrous filler, e.g., glass fiber, carbon fiber, inorganic particulate material, e.g., talc, kaolin, mica, calcium carbonate, wollastonite, and layer structure compound, e.g., mica, montmorillonite, swelling fluoromica with a resin. Some of these materials have been used as various parts in the art of general-purpose consumers' products such as packaging material and vessel, automobile, electricity, electronics, machinery, industry, office machine, space, aviation, etc.
These related art techniques are advantageous in that the resulting molded products exhibit some improvement in strength and rigidity but are disadvantageous in that the products mixed with an inorganic fibrous filler such as glass fiber and carbon fiber exhibit an increased specific gravity, deteriorated external surface appearance or surface smoothness or lowered toughness. These related art techniques are also disadvantageous in that abrasion occurs with the cylinder, screw, mold, etc. in the extruder or molding machine during extrusion or molding. These related art techniques are further disadvantageous in that when subjected to recycling or reworking as required due to the recent growing environmental demand, the resulting products exhibits a drastic deterioration in physical properties such as strength and thus cannot be reused. Further, when a layer structure compound such as mica, montmorillonite and swelling fluoromica is used, the resulting molded products exhibit a drastically reduced weld strength, occasionally making it difficult to use as industrial materials.
Therefore, resin compositions having well-balanced physical properties, high strength, rigidity and heat resistance and excellent toughness, retention of weld strength, dimensional characteristics, external surface appearance and reworkability suitable for various machine industrial parts and industrial materials such as electrical and electronic parts which are different from the related art techniques have been industrially desired.
In order to solve the aforementioned problems, the present inventors proposed a resin composition made of a polyamide resin and finely divided apatite and found a polyamide which exhibits an enhanced rigidity and strength without impairing its toughness as disclosed in WO2000-11099. However, as a result of studies by the present inventors, it was found that since the finely divided apatite is spherical, the resulting polyamide leaves something to be desired in application to various parts such as automobile parts, electric and electronic parts and industrial machine parts.
On the other hand, as a composition made of a resin and apatite there has been disclosed a polyester composition made of a polyester resin and hydroxyapatite having an average primary particle diameter of from 5 to 200 nm, an average secondary particle diameter of from 0.1 to 10 μm and a secondary particle diameter relative standard deviation of not greater than 0.95 in JP-A-2000-119495. Further, a composition made of a polymer resin and fibrous hydroxyapatite having an average fiber diameter of from 5 to 20 μm and an average fiber length of from 100 μm to 5 mm is disclosed in JP-A-63-132810. Referring to the characteristics of the apatites taught therein, the former is finely divided but has no reference to the average aspect ratio of apatite while the latter has a high aspect ratio but a great average diameter.
The present inventors' studies show that the more applicable to industrial parts these compositions are, the more insufficient are their strength, rigidity and heat resistance and the more are ill-balanced their toughness, retention of weld strength, external surface appearance and reworkability.
Referring to finely divided acicular apatite having a high aspect ratio, Masahiro Yoshimura et al. disclose a method which comprises subjecting a low crystallinity apatite with additives such as ethylenediaminetetraacetic acid added thereto to hydrothermal synthesis in Journal of The Chemical Society of Japan, 1991, (10), pp. 1,402–1,407. However, the present inventors' studies show that this method has a remarkably low yield and thus can be difficultly used on an industrial basis.
Therefore, in the related art techniques, no resin compositions made of a resin and finely divided acicular apatite having a high aspect ratio are known, and their characteristics have been unknown.
An object of the present invention is to provide a resin composition comprising a resin and apatite excellent in strength, rigidity and heat resistance as well as in percent weld strength retention, dimensional properties, external surface appearance and reworkability, wherein referring to the particulate form of said apatite present in the resin composition thus obtained, the apatite is present in a particulate form having an average diameter or average thickness (d) of not greater than 100 nm and an average aspect ratio (L/D) of not smaller than 5 as defined by the ratio of the average length (L) to said average diameter or average thickness (d).